Abstract The thymus is the primary immune organ responsible for the generation of T cells, and its functional decline with age (involution) is considered to be a significant contributor to immunosenescence. Despite its central role in the formation and maintenance of adaptive immunity, surprisingly few molecular regulators of thymus organ development and function have been identified, and the molecular basis for aging-associated involution remains unclear. The transcription factor FOXN1 is a key regulator of both thymus organogenesis and postnatal maintenance, and its transcriptional decline precipitates thymic involution. However, the pathways through which it acts and the mechanisms of its regulation remain largely unknown. Forward genetic screens in mice have proven to be a powerful approach for identifying new regulators of developmental and other processes, and have the strong advantage of identifying genes based on their function, allowing unbiased discovery of novel pathways and mechanisms. Furthermore, recent advances in methods for identifying the mutated genes in these screens have significantly reduced the time to gene identification, making small-scale, focused mutagenesis screens an increasingly powerful and useful tool. In the field of aging biology, however, the expense of keeping large numbers of animals to age and the temporal problem of identifying phenotypes before mice become too old to breed has prohibited the use of this powerful approach. My lab has generated a mouse model that presents a novel opportunity for such a screen. Foxn1lacZ/lacZ mice mirror the degree of normal Foxn1 down regulation with aging, and the involution observed is phenotypically similar to aging-associated involution, but is accelerated such phenotypes are observable and quantifiable as early as 2 weeks postnatal, and 2 month-old mice have thymus phenotypes similar to 1-year-old wild-type mice. The Foxn1lacZ/lacZ mice thus present a unique opportunity to perform a forward genetic screen for. In this R21 application, we propose a pilot dominant modifier screen for novel mutations that act as either dominant enhancers or suppressors of the Foxn1lacZ/lacZ phenotype. This screen has the potential to identify new modulators of thymus involution, either dependent on or independent of the action of FOXN1. In the course of this pilot screen, we will assess the feasibility of identifying mutants that modify this phenotype, and test whether those genes identified have normal roles in thymus development, maintenance, and/or involution.